Explicitly tell the frontend to enable default cross-module-optimization

So far, the swift-frontend decided by itself if CMO can be enabled. This caused problems when used with an old driver, which doesn't consider CMO.
Now, the driver decides when to use default CMO.

Author: eeckstein

Sources/SwiftDriver/Jobs/CompileJob.swift

@@ -283,7 +283,13 @@ extension Driver {
     try addCommonFrontendOptions(commandLine: &commandLine, inputs: &inputs)
     // FIXME: MSVC runtime flags
 
-    addDisableCMOOption(commandLine: &commandLine)
+    if Driver.canDoCrossModuleOptimization(parsedOptions: &parsedOptions) &&
+       // For historical reasons, -cross-module-optimization turns on "aggressive" CMO
+       // which is different from "default" CMO.
+       !parsedOptions.hasArgument(.CrossModuleOptimization) {
+      assert(!emitModuleSeparately, "Cannot emit module separately with cross-module-optimization")
+      commandLine.appendFlag("-enable-default-cmo")
+    }
 
     if parsedOptions.hasArgument(.parseAsLibrary, .emitLibrary) {
       commandLine.appendFlag(.parseAsLibrary)

Sources/SwiftDriver/Jobs/EmitModuleJob.swift

@@ -99,8 +99,6 @@ extension Driver {
 
     addCommonModuleOptions(commandLine: &commandLine, outputs: &outputs, isMergeModule: false)
 
-    addDisableCMOOption(commandLine: &commandLine)
-
     try commandLine.appendLast(.emitSymbolGraph, from: &parsedOptions)
     try commandLine.appendLast(.emitSymbolGraphDir, from: &parsedOptions)
     try commandLine.appendLast(.includeSpiSymbols, from: &parsedOptions)
@@ -147,10 +145,9 @@ extension Driver {
                                    default: true)
 
     case .singleCompile:
-      // Non library-evolution builds require a single job, because cross-module-optimization is enabled by default.
-      if !parsedOptions.hasArgument(.enableLibraryEvolution),
-         !parsedOptions.hasArgument(.disableCrossModuleOptimization),
-         let opt = parsedOptions.getLast(in: .O), opt.option != .Onone {
+      // If cross-module-optimization is done, the module must be emitted in the same
+      // swift-frontend invocation which also produces the binary.
+      if canDoCrossModuleOptimization(parsedOptions: &parsedOptions) {
         return false
       }
 
@@ -163,4 +160,13 @@ extension Driver {
       return false
     }
   }
+  
+  static func canDoCrossModuleOptimization(parsedOptions: inout ParsedOptions) -> Bool {
+    if !parsedOptions.hasArgument(.enableLibraryEvolution),
+       !parsedOptions.hasArgument(.disableCrossModuleOptimization),
+       let opt = parsedOptions.getLast(in: .O), opt.option != .Onone {
+      return true
+    }
+    return false
+  }
 }

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -315,14 +315,6 @@ extension Driver {
                                                            driver: self)
   }
 
-  /// Add options to disable cross-module-optimization.
-  mutating func addDisableCMOOption(commandLine: inout [Job.ArgTemplate]) {
-    if parsedOptions.hasArgument(.disableCrossModuleOptimization) {
-      commandLine.appendFlag(.Xllvm)
-      commandLine.appendFlag("-sil-disable-pass=cmo")
-    }
-  }
-
   mutating func addFrontendSupplementaryOutputArguments(commandLine: inout [Job.ArgTemplate],
                                                         primaryInputs: [TypedVirtualPath],
                                                         inputsGeneratingCodeCount: Int,

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -2976,31 +2976,38 @@ final class SwiftDriverTests: XCTestCase {
       var driver = try Driver(args: ["swiftc", "foo.swift", "bar.swift", "-module-name", "Test", "-emit-module-path", rebase("Test.swiftmodule", at: root), "-c", "-o", rebase("test.o", at: root), "-wmo", "-O" ])
       let plannedJobs = try driver.planBuild()
       XCTAssertEqual(plannedJobs.count, 1 + autoLinkExtractJob)
+      XCTAssertTrue(plannedJobs[0].commandLine.contains(.flag("-enable-default-cmo")))
     }
 
     do {
       // library-evolution builds can emit the module in a separate job.
       var driver = try Driver(args: ["swiftc", "foo.swift", "bar.swift", "-module-name", "Test", "-emit-module-path", rebase("Test.swiftmodule", at: root), "-c", "-o", rebase("test.o", at: root), "-wmo", "-O", "-enable-library-evolution" ])
       let plannedJobs = try driver.planBuild()
       XCTAssertEqual(plannedJobs.count, 2 + autoLinkExtractJob)
+      XCTAssertFalse(plannedJobs[0].commandLine.contains(.flag("-enable-default-cmo")))
+      XCTAssertFalse(plannedJobs[1].commandLine.contains(.flag("-enable-default-cmo")))
     }
 
     do {
       // When disabling cross-module-optimization, the module can be emitted in a separate job.
       var driver = try Driver(args: ["swiftc", "foo.swift", "bar.swift", "-module-name", "Test", "-emit-module-path", rebase("Test.swiftmodule", at: root), "-c", "-o", rebase("test.o", at: root), "-wmo", "-O", "-disable-cmo" ])
       let plannedJobs = try driver.planBuild()
       XCTAssertEqual(plannedJobs.count, 2 + autoLinkExtractJob)
+      XCTAssertFalse(plannedJobs[0].commandLine.contains(.flag("-enable-default-cmo")))
+      XCTAssertFalse(plannedJobs[1].commandLine.contains(.flag("-enable-default-cmo")))
     }
 
     do {
       // non optimized builds can emit the module in a separate job.
       var driver = try Driver(args: ["swiftc", "foo.swift", "bar.swift", "-module-name", "Test", "-emit-module-path", rebase("Test.swiftmodule", at: root), "-c", "-o", rebase("test.o", at: root), "-wmo" ])
       let plannedJobs = try driver.planBuild()
       XCTAssertEqual(plannedJobs.count, 2 + autoLinkExtractJob)
+      XCTAssertFalse(plannedJobs[0].commandLine.contains(.flag("-enable-default-cmo")))
+      XCTAssertFalse(plannedJobs[1].commandLine.contains(.flag("-enable-default-cmo")))
     }
 
     do {
-        // Don't use emit-module-separetely as a linker.
+      // Don't use emit-module-separetely as a linker.
       var driver = try Driver(args: ["swiftc", "foo.sil", "bar.sil", "-module-name", "Test", "-emit-module-path", "/foo/bar/Test.swiftmodule", "-emit-library", "-target", "x86_64-apple-macosx10.15", "-wmo", "-emit-module-separately-wmo"],
                                env: envVars)
       let plannedJobs = try driver.planBuild()